Stevia glycosides are a natural sweetener extracted from the leaves and stems of Stevia rebaudiana Bertoni, with an intense sweetness substantially greater than ordinary sugar (sucrose), yet being low in calories. The eight diterpene glycosides, namely, stevioside, rebaudiosides A, B, C, D and E, and dulcosides A and B, are the main ingredients responsible for the sweet taste, of which, stevioside accounts for about 66% of the stevia glycosides, Rebaudioside A accounts for about 22%, rebaudioside C accounts for about 9%, dulcoside A accounts for about 2%, and the other glycosides are present in only trace amounts.
Ordinary stevia glycosides have certain drawbacks, one of which is a prolonged aftertaste. Among the stevia glycosides, Rebaudioside A has the highest sweetness level, at about 300 to 450 times the sweetness of sucrose, and without any undesirable aftertaste. Hence, Rebaudioside A is an ideal natural sweetener. However, other stevia glycosides have very similar chemical structures as they all share the same diterpene skeleton and slightly differ only in the types, quantities, and structural patterns of glycoside moieties. As a result, it is still difficult to obtain high purity Rebaudioside A at an industrial scale.
To date processes for preparing high purity Rebaudioside A include high-performance liquid chromatography, thin layer chromatography, drop counter-current chromatography, capillary electrophoresis, and supercritical fluid extraction. Due to small handling capacity and high cost, these processes are not suitable for industrial production.
Chinese Patent No. 1132840C discloses a method for purifying stevia glycosides through solid-liquid separation. In this process, stevia glycosides and ethanol are mixed at a ratio of 1:1.4-1.7 and dissolved with stirring. The resulting solution is subjected to solid-liquid separation, the solids and liquid obtained from which are decolorized and then dried to obtain purified stevia glycosides. The content of rebaudiosides in the stevia product obtained from this process is 88%, and the product still contains undesirable aftertaste and high levels of impurities.
Chinese Patent No. 1098860C discloses a process for separating enriched stevia glycosides through vacuum filtration and ion exchange resin. The process uses a crude stevia glycosides source material with a ratio of Rebaudioside A to stevioside ranging from 0.5 to 1.1. The crude stevia glycosides is dissolved by stirring the crude with an ethanol-water solvent. The mixture is then filtered to obtain a filtrate, the filtrate is desalinated with ion exchange resin and decolorized with active charcoal. The ethanol and water are evaporated to obtain a stevia product with a yield between 35 to 65% and with a ratio of Rebaudioside A to stevioside greater than 2.5. The content of stevia glycosides obtained by this process is greater than 95%, of which, the content of rebaudioside is equal or greater than 78%.
Chinese Patent No. 1078217C discloses a method for separating enriched Rebaudioside A through adsorption resins. The invention synthesizes a series of macroporous adsorption resins and uses the selective adsorption of macroporous adsorption resins to separate stevia glycosides, and through recrystallization, a stevia product containing 90% Rebaudioside A can be obtained.
Chinese Patent No. 1032651C discloses a method for purifying stevia glycosides through extraction. In this process, fatty alcohol or fatty alcohol plus diluent is used in a liquid-liquid extraction of crude stevia glycosides solution, after which water is used for reverse extraction. The extract obtained from the reverse extraction is then dried to obtain a product with a total content total stevia glycosides at 90%. This process consumes a large amount of test reagents and has poor separation, therefore making large scale production difficult.
Chinese Patent No. 1024348C discloses a process of extracting stevia glycosides through resins. This process includes steps such as soaking the dried stevia plant leaves, plate filtration, adsorption, desalination, decolorization with ordinary strongly alkaline resin, concentration and drying. The content of stevia glycosides in the product obtained from the process is 98%. The process has poor decolorization efficiency, however, and can even be poisonous during use.
Japanese Patent No. 07, 143, 860 discloses a method to manufacture high-content Rebaudioside A. This process uses water or aqueous solvent to extract stevia glycosides from the dried stems or leaves of stevia plant. After separation and collection, an extract with a Rebaudioside A content of 4.8 to 8.5 times the total amount of stevioside, rebaudioside C and dulcoside A can be obtained. Through further crystallization and recrystallization with ethanol containing 10 to 20% of water, a product with 90% Rebaudioside A can be obtained.
Japanese Patent No. 2002,262,822 discloses a sweetener extracted from dried leaves of stevia plants and its extraction method. This process uses water or aqueous solvent to extract stevia glycosides from the dried leaves. In the obtained product, the content of Rebaudioside A is 2.56 times the amount of stevioside.
United States Patent No. 20060134292 discloses a method of extracting sweet glycosides from Stevia rebaudiana Bertoni plants and obtaining Rebaudioside A and stevioside. The extraction is developed in the presence of pectinase, and is further purified using cyclodextrin and bentonite. Purified Rebaudioside A can be obtained by crystallization and recrystallization with ethanol. Purified stevioside can be obtained by purification with cyclodextrin, bentonite, and ion exchange resins. The enzymatic modification of the Rebaudioside A, stevioside and the purified extract is carried out using transferring enzymes derived from Thermoactinomyces vilgaris and Bacillus halophilus. 
United States Patent No. 20060083838 discloses a method for producing a product with high contents of Rebaudioside A. This method uses an ethanol solvent and a stevia starting materials for reflux distillation. Then filtration is done to collect solid residues, which are then washed with ethanol with stirring and filtered again. The solid residues are collected, removed of ethanol and dried to obtain a product with a content of Rebaudioside A up to 99%.
United States Patent No. 20060083838 allegedly teaches a process of obtaining a product with a Rebaudioside A content of more than 99%, but such a result is not reproducible. Repeated trials showed the disclosed steps cannot adequately separate Rebaudioside A from other stevia glycosides, which have very similar structures.